Because void elves didnt evolve by long term exposure to magic source as their ancestors. That is not evolution, but instant transformation, that was created by ethereal and was interrupted in half of the process. Yeah totally the same.. But asking you for the third time, how they recreate that transformation and make new void elves, would be futile, right?
You just assume that there are recruits who would willingly undergo this transformation, so they can be half ethereals who are doomed to die by void madness..Assuming anyone can recreate this transformation and interrupt it in half so they are perfect void elves, not some mishapen void blob.